Working fluid is used in a steel working line as represented by a steel rolling line, and iron powder, which is a magnetic matter, is present in this working fluid in a large amount as a contaminant. The working fluid is used in a large amount in the line and this large amount of working fluid used in the line will be used again in the line after the magnetic matter is removed. That is, the working fluid is circulated and used in the line.
Meanwhile, in the line, the concentration of the magnetic matter in the working fluid is kept at or below a predetermined value to maintain a certain level of steel workability and thus ensure a certain level of product quality. In a conventional practice, a magnetic matter above a predetermined particle size (e.g. a diameter of 50 μm) is removed from the working fluid by processing the working fluid with a filter. Further, a magnetic matter in the form of fine particles with the predetermined particle size and below is removed from this filtered working fluid by processing the working fluid with a magnetic-matter removing apparatus using magnets.
For example, Patent Literature 1 discloses an apparatus for removing iron scum in rolling oil in which: a chain conveyor equipped with magnet plates configured to attract iron scum is provided in such a way as to move the magnet plates in the rolling oil stored in a rolling oil tank and also obliquely pass the magnet plates through the surface of the rolling oil; and a scraper configured to scrape the iron scum attracted to the magnet plates is provided outside the rolling oil tank.